1. Field of the Invention
The present invention relates to polymerization of olefinic monomers. In particular, the instant invention provides a novel catalyst system that incorporates a modifier to improve polymerization results. Additionally, the present invention relates to a novel polymerization process that utilizes the novel catalyst system.
2. Description of the Related Art
The current xe2x80x9cstate of the artxe2x80x9d catalysts for the synthesis of polyolefins are metallocenes. These initiators consist of a Group IV or similar metal dihalide or dialkyl ligated by two cyclopentadienyl fragments or equivalents, and a Lewis Acid activator or cocatalyst, typically methylaluminoxane or a borane, which converts the Group IV complex to its cationic form. Metallocenes are versatile initiators because chemical modification of their cyclopentadienyl (or equivalent) ligands may be carried out in order to tailor catalyst activity and polymer properties. Metallocenes are also extremely efficient initiators, producing one ton or more of polyethylene (PE) per gram of initiator in some extreme cases. However, they are relatively expensive, and preparation of the cyclopentadienyl-type ligands and catalysts may require complicated synthetic procedures. For these reasons, polyolefin catalysts composed of simpler and cheaper organometallics are greatly desired.
Recent research has revealed that catalysts based on main group (Group 13) metals, rather than transition metals, may be used for the synthesis of polyolefins. Jordan (Polym. Mat. Sci. Eng., 1999, 80, 418-419; J. Am. Chem. Soc., 1997, 119, 8125-8126; J. Am. Chem. Soc., 1998, 120, 8277-8278; U.S. Pat. No. 5,777,120) and Gibson (Chem. Commun., 1998, 2523-2524; Chem. Commun., 1999, 1883-1884) have both reported catalysts for the preparation of linear polyethylene based on borane-activated, cationic LAlR+ species (where L=a ligand; Al=aluminum; and R=an alkyl or hydride group). These catalysts possess only low activities and require complex, chelating N,Nxe2x80x2- or N,Nxe2x80x2,Nxe2x80x3-spectator ligands to protect the cation from deactivating side reactions. In 1992, Martin (Makromol. Chem., 1992, 193, 1283-1288) reported the preparation of high-molecular weight polyethylene using simple neutral R3Al or (Cl2Al)2Et compounds as initiators. In 1998, Sen (Polym. Prepr., Am. Chem. Soc. Div. Polym. Chem., 1998, 39(2), 510; J. Am. Chem. Soc., 2000, 122, 5668-5669; U.S. Pat. No. 6,291,387) developed an improved methodology involving neutral alkylaluminum catalysts that are activated for polymerization, but not rendered cationic, by the addition of a borane or methylaluminoxane (MAO) in the highly polar, noncoordinating solvent chlorobenzene. Under conditions of 50xc2x0 C. and 800 psi ethylene, linear polyethylene (Tm=138-140xc2x0 C.) is obtained. The molecular weights of the resultant polymers may exceed 200,000 and polymerization activities can reach 13.75 kg PE/mol catxe2x80xa2h, the highest values achieved of any of the aluminum-based systems discussed above. Propylene may also be homo- or co-polymerized.
Ethylene polymerization catalysts composed of simpler and cheaper organometallics as compared to metallocenes are continually sought by manufacturers of polyolefins. Recent reports of non-transition-metal-containing polymerization initiators based on aluminum, specifically xe2x80x9cactivated neutralxe2x80x9d rather than cationic aluminum, are attractive for these reasons. See U.S. Pat. No. 6,291,387 to Sen, et al. But these initiators possess much lower polymerization activities than metallocenes. For these initiators to be considered as potential economically attractive replacements to metallocenes, their activities must be improved.
Electron-donating olefins have been used in PCT Patent No. WO 97/27224 as catalyst modifiers for ethylene/1-hexene and propylene polymerizations carried out with solid-supported metallocenes/MAO. Moderate improvements in activity and 1-hexene comonomer incorporation were observed. U.S. Pat. No. 5,912,202 to Oskam, et al., discloses the addition of 1-hexene to single site Group IV catalyst precursors to improve ethylene/1-hexene copolymerization activities. In Canadian Patent No. CA 914,849 to Wristers, alpha-monoolefins were utilized to improve the efficiency of Ziegler-Natta catalysts systems; however, the use of cyclopentene and other more highly substituted olefins was not effective. No work has been carried out involving the use of additives or modifiers for main-group-mediated, transition metal-free olefin polymerizations.
The instant invention describes the use of activity-enhancing modifiers in a transition metal-free catalyst system for the improved polymerization of olefinic monomers. The modifiers react with an AlR3 compound, wherein Al is aluminum and R is an alkyl or hydride group, and a Lewis acid or Lewis acid derivative cocatalyst. The modifiers are internal, cyclic, or terminal olefins. The most preferred modifiers are cyclopentene, cyclohexene, norbomene, cis-stilbene, trans-stilbene, cis-2-hexene, trans-2-hexene, cis-3-hexene, trans-3-hexene, styrene, and 1-hexene.
The catalyst system can be either supported or not supported. When supported, the catalyst system preferably is a silica supported system.
The present invention is also directed to a method for polymerizing olefinic monomers using the novel transition metal-free catalyst system comprising an AlR3 compound, a Lewis acid or Lewis acid derivative cocatalyst, and an activity-enhancing modifier.
The preferred olefinic monomers are ethylene, propylene, and an ethylene-propylene combination.
The polymerization method may be conducted in a solvent. The preferred solvent is a polar solvent, most preferably chlorobenzene.